1. Field of the Invention
The present invention relates to a method of manufacturing steel sheets and more particularly to a descaling method of removing oxide scales formed on the surface of hot-rolled steel strips at high speed and an apparatus thereof.
2. Description of the Prior Art
Generally, steel sheets (carbon steel) are rolled at temperatures of 800.degree.-900.degree. C. and thus black oxide scales mainly of Fe.sub.3 O.sub.4 form on the their surfaces. The scales may damage the surface of the steel sheet in the subsequent cold rolling process and therefore must be removed. As the demand for automotive thin steel sheet is currently increasing, the descaling at high speed is becoming increasingly important.
The conventional descaling methods for steel strips are classified largely into a chemical and a mechanical method. The mainstream of the chemical method is a catenary pickling method, which involves immersing and continuously passing steel plates through tanks of acid solution to remove scales from the steel sheets by chemical reactions. Among methods having an enhanced efficiency of descaling oxides by immersing in acid solution are a box dam method and a jet flow method. The box dam method consists in filling a rectangular parallelepiped container with acid solution and passing steel plates through it for pickling. The rectangular parallelepiped container has weirs installed at the upper and lower surfaces thereof. The jet flow method, as described in the Mitsubishi Juko Giho (Mitsubishi Heavy Industry Technique) Vol. 29, No. 1 (1992-1), has a jet nozzle installed in the box dam structure, whereby acid solution is sprayed against the steel sheet to further enhance the oxide scale removing effect.
The mechanical descaling methods include a rolling method, a polishing method, a shot blast method and a repetitive bending method, all introduced in the Hitachi Hyoron (Hitachi Review) Vol. 67, No. 4 (1985-4) as new technologies for high-speed descaling facilities. Also available are a high-pressure water spray method, a mechanical and pickling combination method, and an ultrasonic pickling method introduced in the Mitsubishi Juko Giho Vol. 2, No. 3, p. 289 (1965).
The conventional pickling for steel strips uses a dilute hydrochloric acid (HCl) because iron oxides easily dissolve in it. The reactions that occur are expressed by EQU Fe.sub.2 O.sub.3 +6HCl.fwdarw.2FeCl.sub.3 +3H.sub.2 O (Reaction 1) EQU Fe.sub.3 O.sub.4 +8HCl.fwdarw.FeCl.sub.2 +2FeCl.sub.3 +4H.sub.2 O(Reaction 1)
To speed up the descaling of oxides, the reactions (1) and (2) need be accelerated. Because this method utilizes a chemical dissolution reaction of the oxide scale, a generally conceivable method for efficient descaling may be by increasing the acid concentration and temperature to accelerate the reaction. The increase in the acid concentration and temperature, however, is in reality restricted by a cost of acid disposal processing, problems involving environment and facility, and the surface quality, and it is difficult to increase the acid concentration and temperature from the current level. The pickling process involves continuously immersing the steel strips in a plurality of acid baths. In the first tank of acid solution it is difficult to raise the temperature of the steel strips to a sufficiently high level. Moreover, the first tank has the lowest acid concentration. Because of these factors and a time lag before the scale dissolution begins, the first tank has an inherent problem of extremely low pickling efficiency.
To increase the pickling speed, improvements have been made over the conventional box weir method and jet flow method, whereby agitation is introduced to reduce the temperature boundary layer and thereby accelerate heat conduction to steel sheets and at the same time replenish a liquid close to the steel sheet surface efficiently. These improved methods, however, are still unable to raise the oxide scale removing efficiency at the initial stage of the pickling and it is difficult to improve the speed of descaling the oxides. In either case, when the speed at which the steel sheets are passed through the acid solution is low, the oxide scales can be removed. But as the pass-through speed is increased, the perfect descaling cannot be obtained.
Other conventional techniques include a method in which an electrolysis is performed to a stainless steel in a sulfuric acid, nitric acid, neutral salt or molten salt to increase the dissolution speed. Because the oxide scale of the steel strip is about two orders of magnitude thicker than that of the stainless steel, the direct application of a technique developed for the stainless steel to the steel strip cannot remove the oxide scales completely. In the case of the stainless steel, for the purpose of accelerating the dissolution of chrome oxide, the indirect current application method has a major part of the stainless steel strip used as an anode and minimizes the cathode portion that does not accelerate the dissolution reaction. This is because the cathode reaction causes the chrome oxide to precipitate again. At the anode, however, a base material may also dissolve. If these methods are applied to the removing of oxide scales from the steel strips, the surface of the steel strip cannot be kept smooth, making it impossible to manufacture high-quality products.